The present invention relates to a lip seal device having a lip seal in sliding contact with a shaft. A lip seal device having a plurality of lip seals has been well known. Such a lip seal device usually comprises a cylindrical case, and first and second lip seals whose outer circumferential portions are attached to the cylindrical case and whose inner circumferential portions are curved toward a sealed fluid and put in sliding contact with the outside circumferential surface of a shaft extending through the cylindrical case. The first lip seal is located nearer the sealed fluid than the second lip seal so that the sealed fluid having leaked along the first lip seal is blocked by the second lip seal. The first and the second lip seals have substantially the same constitution and have their inner circumferential portions put in sliding contact with the shaft by tight contact forces of substantially the same magnitude. Nagasawa. U.S. Pat. No. 4,623,153, shows such a lip sealing device having two lip seals. However. Nagasawa does not distinguish between the contact pressure of the respective lip seals, as does the subject invention.
Since there is a large fluid pressure difference between the two sides of the first lip seal, which is located nearest the sealed fluid, the first lip seal needs to be in sliding contact with the shaft by a strong contact force. In contrast, since the second lip seal is for blocking the sealed fluid that has leaked along the first lip seal, the second lip seal does not need to be in sliding contact with the shaft by a strong contact force. Typically, the second lip seal is in sliding contact with the shaft by more than a necessary contact force. As a result, the sliding contact resistance of the second lip seal to the rotation of the shaft is increased, and the life of the second lip seal is shortened. These are drawbacks of the prior devices.